Step 1: Make a new world. A flat world with plenty of trees and materials helps.
Step 2(optional but helps): Go to World Laws and turn off Natural Disasters, Old Age, Hunger, Peaceful Monsters, and anything that lets wars happen. Make sure kingdom expansion is on though.
Step 3: Go to settings and press the burger 11 times (top left corner. if there's a dragon there press the gear in the middle then burger should show up). There should now be a green bug thing in the top right. Press it. Scroll down and press the Greg button.
Step 4: Spawn 1 human. Wait for him to make a village and then spawn humans inside the village borders (hold shift for fast spawn) until the village has 1000 or more people.
Step 4(optional but helps): Go back to the green bug thing. Turn on sonic speed. Scroll down until you see a category of buttons like fastPopGrowth and unlimitedResources. Turn on all in that category.
Step 5: Wait until the kingdom has 3 or more villages. Make sure at least one village has a mine. The more mines there are the better. Ideally you want a mine in every village.
Step 6: Go to the Ages menu and turn off all ages except the Age of Despair and set the Current Age to Age of Despair.
Step 7: Wait. Make sure Sonic Speed is on otherwise it might take a while. Eventually you should get a message in the top left that says, "Oh no! The fear of the inhabitants of this world has embodied in a physical form. The horror!" When you do, pause the game, turn off Sonic Speed and check all the mines, because Greg always spawns at a mine.
Congrats you now should get the Greg achievement!

Requirements for Greg:
1: Greg turned on in Debug menu
2: Other Disasters turned on in World Laws
3: 1000+ population in your world
4: 3+ villages in one kingdom with at least one mine
5: Current age is Age of Despair

If you use a workshop map that already has Greg, you won't get the achievement. You need to be in the world when he spawns.

Hope this helped have fun with Greg